A common two-stage operational amplifier includes an input stage and a second stage. The input stage is usually a differential amplifier with a transconductance characteristic, producing an output current from the differential voltage inputs. Transconductance is typically very high, and thus a fairly small input voltage may be sufficient to cause the input stage to saturate, and accordingly produce a constant current output.
The current output from the input stage is received by the second stage, where frequency compensation is accomplished. The low pass characteristic of this stage approximates an integrator, and a current input which can therefore produce a linearly increasing output if current input is constant. The input current to the second stage along with the compensation capacitance and gain-bandwidth of the amplifier all affect the slew rate of the two-stage operational amplifier.
The slew rate of an amplifier represents the maximum rate of change of a signal at any point in the circuit. In other words, limitations in slew rates may result in non-linear effects that may significantly distort amplifier output if the input signal is at a frequency exceeding the slew rate limitations of the amplifier. As such, the frequency of the input signal is often limited by the capabilities of an amplifier during circuit design.